1. Field of the Invention
The present invention relates to a controller driver for driving a liquid crystal panel, a display apparatus, and a driving method of a liquid crystal panel.
2. Description of Related Art
Portable information equipment such as mobile phones and PDA includes a controller driver for driving a liquid crystal panel. Some controller drivers have an image memory capable of storing image data of one frame and a simple controller for generating a synchronization signal to indicate a display timing of the image data stored in the image memory. In this configuration, if there is no need to switch display images such as when displaying a still image, it is possible to display a still image by displaying the image data stored in the image memory on a liquid crystal panel without receiving image data from an external processor such as CPU. Such a configuration is effective for reducing power consumption.
FIG. 14 shows an example of a conventional liquid crystal display apparatus that has a controller driver with a built in memory. The conventional liquid crystal display apparatus includes a liquid crystal panel 7, a gate line driver 6 for driving a gate line of the liquid crystal panel 7, and a controller driver 8 for receiving image data Dn from a processor 5 such as CPU and displaying it on the liquid crystal panel 7 of a mobile phone terminal or the like. The controller driver 8 includes an image memory 83 capable of storing image data of at least one frame, a tone voltage generator 17 for generating a tone voltage, a data line driver 89 for driving a data line of the liquid crystal panel 7, a timing controller 18 for indicating the data line driver 89 and the gate line driver 6 of a display timing, and a command controller 80 for indicating the tone voltage generator 17 of a setting of a tone voltage and indicating the timing controller 17 of an image display timing and so on. The configuration of the controller driver 8 shown in FIG. 1 is merely an example, and a controller driver may include a gate line driver or may further include a power supply circuit.
As described above, since the controller driver 8 has the image memory 83 capable of storing image data of at least one frame, it is possible to display a still image that is stored in the image memory 83 on the liquid crystal panel 7 without a need to transfer image data from the external processor 5. Specifically, the command controller 80 indicates the image memory 83 to transfer image data to the data line driver 89 and further indicates the data line driver 89 and the gate line driver 6 of a timing to display the image. This configuration allows stopping the operation of the external processor 5 during still image display and thereby reducing power consumption.
As mobile phone terminals become highly functional, they are required to have a function to display moving images. However, a liquid crystal panel has a slow speed of response to a change in display images, which causes an image out of focus when displaying moving images. To overcome this drawback, overdrive processing is performed in a large-sized liquid crystal panel or the like in order to improve a response speed of liquid crystal. The overdrive processing compares present image data with one frame previous image data. If a tone increases and thus luminance is higher, it drives a liquid crystal panel with a higher liquid crystal driving voltage than a normal level. If, on the other hand, a tone decreases and thus luminance is lower, it drives a liquid crystal panel with a lower driving voltage than a normal level. This processing increases a response speed of a liquid crystal panel. The overdrive processing is detailed in Japanese Patent No. 2616652, Japanese Unexamined Patent Publication No. 4-365094 and 2003-202845, for example.
Adding an overdrive processor to the controller driver 8 with the image memory 83 enables to improve a response speed of liquid crystal. However, there is a restriction in size for portable information equipment such as a mobile phone terminal, and thus a chip size of the controller driver 8 is preferably small. Merely adding the overdrive processor to the controller driver 8 results in an increase in the chip size of the controller driver 8.
As a means to reduce the chip size, it is effective to store image data after compressing it so as to reduce the size of an image memory that occupies a large proportion of a chip area. However, performing the overdrive processing with use of compressed image data stored in the image memory or its expanded image data fails to control a voltage applied to a liquid crystal panel accurately.
For example, in the case of compressing image data by a systematic dither method that is conventionally known, errors that are spatially distributed by the dither processing are enhanced by the overdriving processing, which causes an image displayed on a liquid crystal panel to be more granular. Specifically, if image data is compressed by 2 bits with use of a 2×2 dither matrix, even if input image data have the same tone, computing with the dither matrix results in an image with a four tone difference. It is assumed herein that the overdrive processing is performed when an entire display image changes from the same color to different colors. In this case, excessive overdrive of four tones occurs in some place. In the dither process, the excessive overdrive is applied to a particular place. This leads to more granular image display.